The MoFe protein of nitrogenase is under intense study as the catalytic component of the biological N 2 fixation system. We are using x-ray absorption spectroscopic examination of the Mo site of its Mo-Fe-S cluster active site, both in the intact enzyme and in its isolated form (called FeMoco). These experiments have so far been limited to wild-type enzyme. Recently, we have been able to obtain large quantities of altered forms of the MoFe protein from mutant organisms and have also obtained altered forms of FeMoco. EXAFS analysis of these species should define both the portions of FeMoco that are required for biological activity and the transformation that occurs at the Mo site during FeMoco biosynthesis. We are further studying the effects of substrates and inhibitor on the structure of the active site.